Association of Ontario Midwives
ExpErts in normal prEgnancy, birth & nEwborn carE
Midwiveso n t a r i o
Management of the Uncomplicated PregnancyBeyond 41+0 Weeks’ Gestation february 2010
> Clinical Practice Guideline No.10
Association of Ontario Midwives
365 bloor st. E., suite 301
toronto, on m4w 3l4
www.aom.on.ca
Management of the Uncomplicated Pregnancy
Beyond 41+0 Weeks’ Gestation
> Clinical Practice Guideline No.10
Authors
Julie corey, rm mhsc
tasha macDonald, rm mhsc
Contributors
Clinical Practice Guideline Subcommittee
Elizabeth Darling, rm msc, chair
cheryllee bourgeois, rm
corinne hare, rm
Jenni huntly, rm
paula salehi, rm
lynlee spencer, bsc
Vicki Van wagner, rm, phD (c)
rhea wilson, rm
Insurance and Risk Management Program Steering Committee
‘remi Ejiwunmi, rm, chair
abigail corbin, rm
Elana Johnston, rm
carolynn prior van Fraassen, rm
lisa m weston, rm
AOM Staff
suzannah bennett, mhsc
cindy hutchinson, msc
bobbi soderstrom, rm
Acknowledgements
Kristen Dennis, rm
ontario ministry of health and long-term care
ryerson University midwifery Education program
the association of ontario midwives respectfully
acknowledges the financial support of the ministry
of health and long-term care in the development
of this guideline.
the views expressed in this guideline are strictly
those of the association of ontario midwives. no
official endorsement by the ministry of health and
long-term care is intended or should be inferred.
Statement of purpose:
the goal is to provide an evidence-based clinical practice guideline (cpg) that is consistent with the midwifery philosophy of care. midwives are encouraged to use this cpg as a tool in clinical decision-making.
Objectives:
to provide a critical review of the research literature for women reaching 41+0 weeks’ gestation and beyond in an uncomplicated pregnancy, as well as to provide recommendations regarding management options. Evidence relating to the following will be discussed:
Factors contributing to an increased risk of • postdates pregnancy
Effective interventions for reducing the • rate of postdates pregnancy
impact of postdates pregnancy on • maternal and neonatal outcomes
management options for postdates • pregnancy.
Outcomes of interest:
maternal outcomes: rate of caesarean 1. section, instrumental delivery, morbidity, satisfaction with care
neonatal outcomes: perinatal mortality, 2. perinatal morbidity
Methods:
a search of the medline database and cochrane library from 1994-2009 was conducted using the key words: prolonged pregnancy, postdates pregnancy, postterm pregnancy. additional search terms were used to provide more detail on individual topics as they related to postdates pregnancy: antenatal monitoring, fetal movement counting, evening primrose oil and gestational age calculation. older publications were accessed to include seminal randomized controlled trials, commonly cited sources for incidence rates or studies that had significant impact on clinical practice. Evidence was graded using the canadian task Force on preventive health care grading system. (1)
Review:
this guideline was reviewed using a modified version of the agrEE instrument (2), the aom’s Values-based approach to cpg Development (3), as well as consensus of the aom cpg sub-committee, the insurance and risk management program, and the aom board of Directors.
AOM CliniCAl PrACtiCe GUideline
Management of the Uncomplicated PregnancyBeyond 41+0 Weeks’ Gestation
This guideline was approved by the AOM Board of Directors: February 17, 2010
This guideline reflects information consistent with the best evidence available as of the date issued and is subject to change. The information in this guideline is not intended to dictate a course of action, but inform clinical decision-making. Local standards may cause practices to diverge from the suggestions within this guideline. If practice groups develop practice group protocols that depart from a guideline, it is advisable to document the rationale for the departure.
Midwives recognize that client expectations, preferences and interests are an essential component in clinical decision-making. Clients may choose a course of action that may differ from the recommendations in this guideline, within the context of informed choice. When clients choose a course of action that diverges from a clinical practice guideline and/or practice group protocol this should be well documented in their charts.
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 1
ABBREVIATIONS:
CI – confidence interval
CS – caesarean section
BMI – body mass index
BPP – biophysical profile
EDB – estimated date of birth
LMP – last menstrual period
MAS – meconium aspiration syndrome
MSAF – meconium stained amniotic fluid
NNT – number needed to treat
OR – odds ratio
RR – relative risk
T1 – first trimester
T2 – second trimester
2 Association of Ontario Midwives
Key to evidence statements and grading of recommendations, from the canadian task Force on preventive health care
Evaluation of evidence criteria classification of recommendations criteria
i Evidence obtained from at least one properly randomized controlled trial
a there is good evidence to recommend the clinical preventive action
ii-1 Evidence from well-designed controlled trials without randomization
b there is fair evidence to recommend the clinical preventive action
ii-2 Evidence from well-designed cohort (prospective or retrospective) or case-control studies, preferably from more than one centre or research group
c the existing evidence is conflicting and does not allow to make a recommendation for or against use of the clinical preventive action; however, other factors may influence decision-making
ii-3 Evidence obtained from comparisons between times or places with or without the intervention. Dramatic results in uncontrolled experiments (such as the results of treatment with penicillin in the 1940s) could also be included in this category
c the existing evidence is conflicting and does not allow to make a recommendation for or against use of the clinical preventive action; however, other factors may influence decision-making
iii opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees
D there is fair evidence to recommend against the clinical preventive action
E there is good evidence to recommend against the clinical preventive action
l there is insufficient evidence (in quantity or quality) to make a recommendation; however, other factors may influence decision-making
reference: (1)
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 3
INTRODUCTION
pregnancy at 41+ weeks’ gestation is seen often
in midwifery practice. though it is generally a
normal and healthy occurrence associated with
good outcomes for women and babies, pregnancy
at 41+ weeks has been associated with increased
risks of meconium stained amniotic fluid (msaF),
meconium aspiration syndrome (mas), shoul-
der dystocia and stillbirth. Determining the best
method of calculating an estimated date of birth
(EDb), effective monitoring of fetal well-being, and
if, when and how to intervene to initiate labour are
all important aspects of postdates pregnancy man-
agement. midwives providing care for pregnancy
at 41+ weeks’ gestation aim to avoid unnecessary
intervention while facilitating the best possible
outcomes for mothers and babies.
Discussing and implementing a plan for manage-
ment of pregnancy at 41+ weeks is part of the
informed choice process. in order to facilitate cli-
ent decision-making regarding pregnancy at 41+
weeks, midwives need to be aware of the risks and
benefits of interventions such as induction of la-
bour, as well as of expectant management.
Definition of Terms
according to the internationally recommended
definitions endorsed by the world health organi-
zation (who) and the international Federation of
gynecology and obstetrics (Figo), ‘postterm’ preg-
nancy is defined as pregnancy lasting 42+0 weeks
(≥ 294 days) or more. ‘postdates’ pregnancy is de-
fined as lasting 40+0 weeks plus one or more days
(i.e. anytime past the estimated date of birth), and
‘prolonged’ pregnancy is any pregnancy after 42+0
weeks (or synonymous with postterm). (who
1977, Figo 1986, cited in (4))
considerable confusion arises, however, as ‘post-
term’, ‘postdates’ and ‘prolonged’ pregnancy tend
to be used interchangeably in research literature
and textbooks, as well as by health care provid-
ers. the lack of precision in the use of the terms
associated with pregnancies that pass the EDb is
widespread and may lead to misunderstanding as
described by murray Enkin:
“semantic problems have also contrib-
uted to the confusion in understand-
ing of postterm pregnancy. the words
‘postterm,’ ‘prolonged,’ ‘postdates’ and
‘postmature’ are all used as synonyms
but are laden with different evaluative
undertones.” (5)
Further, this ambiguity in the use of terms associ-
ated with postdates pregnancy makes “accurate
compounding of the qualitative data” difficult. (6)
where possible, the gestational age upon which
research studies based their results will be speci-
fied in this cpg, but due to inconsistencies in the
way data was collected, gathered or reported, this
level of accuracy in reporting outcomes according
to gestational age is not always possible.
Using specific language when communicating
with other health care providers as well as help-
ing clients to understand these terms will improve
clarity when communicating management plans
for postdates pregnancies.
Incidence of Postterm Pregnancy
it is difficult to determine the true prevalence of
postterm pregnancy because inaccurate pregnan-
cy dating tends to over-estimate incidence, and
induction of labour will reduce rates of postdates.
several retrospective studies that applied strict cri-
teria for pregnancy dating have reported rates of
6% to 8% for postterm pregnancy. (7-9) statistics
canada data (1980-1995) showed an increase in
the number of births at 41 weeks (11.9% in 1980
to 16.3% in 1995) and a decrease at 42 or more
weeks (7.1% in 1980 to 2.9% in 1995) reflecting
changes in clinical practice for the management
of postdates pregnancy in canada during this time
period (10) and possibly due to the increasing use
of ultrasound allowing for more accurate pregnan-
cy dating.
Contributing Factors for Pregnancy ≥ 41+0 Weeks’ Gestation
several retrospective cohort studies have identi-
fied factors that contribute to prolonged preg-
nancy. these include high body mass index (bmi),
nulliparity, fetal male gender and a previous post-
term (≥ 42+0 weeks) pregnancy. two cohort stud-
ies found an inverse correlation between the rate
4 Association of Ontario Midwives
of spontaneous labour at term and first trimester
bmi: the chance of having a postterm pregnancy
goes up as bmi increases (or ranging from 1.24
with a bmi of 25-29 to 1.52 with a bmi over 35;
95% ci 1.28-1.82). (8,11)
pregnant women are more likely to have pregnan-
cies lasting ≥ 41 weeks if the fetus is male than if
it is female. a retrospective study reviewed 82 484
singleton births and found that there was a higher
rate of postdates pregnancies when the fetus was
male (rr 1.41 at 42 weeks) and proposed that this
may be due to potential measurement bias (male
fetuses are slightly larger on average and there-
fore ultrasound measurements may tend to inter-
pret them as being at a slightly later gestation).
(12) another review (656 423 births) also found a
higher likelihood of postterm pregnancy when the
fetus was male (or 1.14 at 41 wks, 1.39 at 42 wks,
1.50 at 43 wks) but concluded that measurement
bias does not account for all of the increase and
suggested a gender-related component to the ini-
tiation of labour. (13) some evidence suggests that
women are also more likely to have a postterm
pregnancy when having their first baby (rr 1.35 -
1.46) (9,14) or if they have had a previous postterm
pregnancy (rr 1.38 – 1.88). (4,15,16)
Summary Statement
Nulliparity, high BMI, history of a previous postdates
pregnancy and male fetal gender are all associated
with a higher rate of postterm pregnancy. (II-2)
MATERNAL COMPLICATIONS OF POSTDATES PREGNANCY
several studies have examined the maternal com-
plications associated with postdates including in-
strumental delivery and caesarean section (cs).
a Finnish retrospective population-based cohort
study conducted from 1990-2000 (1678 postterm
pregnancies) found that postterm pregnancy was
associated with an increase in instrumental de-
livery (10.7% vs. 5.3%) (or 1.97, 95% ci 1.06-1.37,
p < .01). (9) background data, obstetrical risk fac-
tors and health behaviours were included in the
analysis to limit the influence of confounding vari-
ables. another retrospective review of 36 160 low
risk pregnancy outcomes from 1989-1997 in israel,
found an increase in cs with increasing gestation-
al age postdates (5.4% at 40 wks, 5.8% at 41 wks,
7.9% at 42 wks, 8.2% at 43 wks). (7) similarly, in a
retrospective cohort study including 119 254 low
risk pregnancies in california in the period 1995-
1999, caesarean rates increased from a rate of
9.0% at 40 weeks to 14.0% at 41 weeks and 21.7%
at ≥ 42 weeks (p < .01). the authors controlled for
length of labour, induction of labour and type of
anaesthesia using multivariate regression. (17) the
above studies have demonstrated an association
with increased rates of cs for postdates pregnan-
cies, rather than demonstrating causation. sim-
ply being aware that a woman is ‘postdates’ may
cause health care providers to intervene more read-
ily (due to labelling). (6)
PERINATAL COMPLICATIONS OF POSTDATES PREGNANCY
perinatal complications associated with pregnancy
≥ 41+0 weeks include meconium stained amniotic
fluid, meconium aspiration syndrome, shoulder
dystocia and stillbirth.
three randomized controlled trials (comprising
3407, 440 and 508 women respectively) found sim-
ilar perinatal mortality rates in those induced at 41
weeks and those managed expectantly who deliv-
ered between 41 and 43 weeks’ gestation. (18-20)
a large retrospective review of 408 631 births be-
yond 41+0 weeks’ gestation from the norwegian
medical birth registry (1999-2005) found the peri-
natal mortality rate increased with increasing ges-
tational age (0.18% at 41 weeks, 5.1% at 43 weeks),
no data was given for 42 weeks. in this analysis,
the number of inductions necessary (nnt) to avoid
one intrauterine fetal death (iUFD) or perinatal
death at 41 weeks was determined to be 671 (95%
ci: 571-794) and 195 at 43 weeks (95% ci: 84-600),
p < .004. the authors note that there is a down-
ward curve in inductions needed to avoid perina-
tal death and iUFD near 286 days gestation (42+2
weeks), see Figure 1. (21) a population-based pro-
spective study of 17 493 singleton pregnancies,
also from norway (1989-1999), with a second-
trimester ultrasound examination and delivery af-
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 5
Figure 1: Numbers of inductions needed to avoid 1 IUFD and 1 perinatal death according to gestational age. Data from the Medical Birth Registry of Norway (21)
ter 37+0 gestational weeks found that the relative
risk for perinatal mortality per 1000 total births de-
creased from week 37+0 until 41+0 weeks, reach-
ing a nadir and then increasing at week 42+0 (rr
1.24 at 37 weeks, 1.0 at 40 weeks, 0.43 at 41 wks
and 1.92 at 42 wks). the absolute risk of perinatal
mortality in this study was similar to the norwe-
gian medical birth registry review above (2.7/1000
at 40 weeks, 1.18/1000 at 41 weeks and 5.23/1000
at 42 weeks). (16)
a retrospective review of 36 160 low-risk pregnan-
cies found an increase in msaF (17.5% at 40 wks,
21.5% at 41 wks, 25% at 42 wks, 37.7% at 43 wks,
p < .001). (7) another cohort study of 32 679 low-
risk, cephalic, singleton births delivered at ≥ 37+0
weeks of gestation (1976-2001) found an increase
in mas (or 2.18 at 40 wks, 3.35 at 41 wks, 4.09 at
42 wks). (22) multivariate analyses were used to
control for ethnicity, weight, age, economic status
and obstetrical history. a retrospective population
based cohort study of 1678 postdates pregnancies
from one hospital in Finland (1990-2000) found
that postterm pregnancy was associated with an
increase in meconium staining (21.2% vs. 12.8%;
p < .01), and shoulder dystocia (4.1% vs. 2.4%,
p < .01), but found no significant difference in peri-
natal mortality or morbidity. (9) it is important to
note that although the risk of complications rise
with increasing gestational age past 41+0 weeks,
the absolute risk of adverse events remains small
(see table 1).
there is evidence that the risk of an adverse out-
come is greater for smaller postterm babies. a
large retrospective study (510 029 singleton term
and postdates births) found that postterm small
for gestational age (sga) babies are at higher risk
of stillbirth, compared with postterm appropriate
for gestational age babies (or 10.56; 95% ci 6.95
- 16.05) and that the rate of sga babies is higher
in the postterm period (3.8%) compared to at term
6 Association of Ontario Midwives
(2.2%). (26,27) a prospective study following 792
pregnancies after 41 weeks reported an inverse
relationship between non-reassuring fetal status
and birth weight category. smaller babies (<10th
percentile) were more likely to have abnormal
findings during antenatal monitoring (36% vs. 14%
of average size babies) and more likely to need a cs
for non-reassuring fetal status (12.3% vs. 5.3% for
average and large size babies, p < .024). while the
authors do not distinguish sga babies from intra-
uterine growth restriction babies with regards to
outcomes, the latter study excluded any previously
suspected growth restricted fetuses. (28)
Summary Statement
After 41+0 weeks’ gestation, the risk of meconium
stained amniotic fluid, meconium aspiration syn-
drome and perinatal mortality rate increases with
increasing gestational age, though the absolute
risks associated with increasing gestational age are
small. (II-2). Perinatal risk seems to be higher for
postterm babies who are also small for gestational
age. (II-2)
PREVENTION OF POSTTERM PREGNANCY
Establishing An Accurate Estimated Date of Delivery
Determining the length of gestation and an accu-
rate estimated date of birth (EDb) can have “pro-
found personal, social, and medical implications.”
(29) there are a variety of methods for assessing
Table 1: Risk of Fetal Complications by Gestational Age: Perinatal Complications per 1000 births
Complication
(Study Country)
Gestational age (weeks)
40 41 42 43
Fetal complication / 1000 births
meconium stained
amniotic Fluid (israel)
n=30 478 (7)
175 215 250 n/a
meconium aspiration
syndrome (norway)
n=27 514 (23)
2.9 5.1 4.7 n/a
neonatal deaths‡/ stillbirths*
(calculated separately by the
authors) (United Kingdom)
n= 171 527 (24)
1.2/1.5
(total=
3.7)
0.7/1.7
(total=
2.4)
1.8/1.9
(total=
3.7)
1.6/2.1
(total=
3.7)
perinatal mortality rate†
(norway)
n=17 493 (16)
2.72 1.18 5.23 n/a
‡Neonatal death (WHO definition): Number of deaths during the first 28 com-
pleted days of life per 1000 live births in a given year or period. Neonatal deaths
may be subdivided into early neonatal deaths, occurring during the first seven
days of life, and late neonatal deaths, occurring after the seventh day but before
the 28 completed days of life. (25)
*Stillbirth: none of the signs of life are present at or after birth (26)
†Perinatal mortality rate (WHO definition): number of stillbirths and deaths in
the first week of life per 1000 live births. (25)
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 7
gestational age and each method has strengths
and weaknesses relating to the prevention of post-
term birth, women’s agency and the judicious use
of technology. therefore the method of EDb calcu-
lation deserves careful consideration.
Decisions on how best to manage a postdates preg-
nancy involve accurate estimates of gestational
age. there is still some controversy as to how ges-
tational age is best estimated during pregnancy.
research examining methods of establishing EDb
have studied differences in the accuracy of using
menstrual dating alone, ultrasound dating at dif-
ferent gestational ages, or algorithms that combine
both menstrual dating and ultrasound biometry.
Length of Human Gestation
the duration of human gestation as calculated
from the first day of the last menstrual period
(lmp), assuming a 28-day menstrual cycle, is often
quoted as 280 days or 40+0 weeks. (26,30-32)the
true length of gestation is the time from concep-
tion until delivery occurs. (30) Empirical evidence
suggests that gestation ranges from 266-274 days
in studies calculating the gestational period from
ovulation; the length from conception is most
commonly cited as 266 days. (32,33)
this 280-day estimate for the duration of human
gestation differs by 2 days from a retrospective
analysis of birth data for 427 581 births in swe-
den in the period from 1976-1980. in this study
examining the duration of pregnancy for singleton
births, analysis of last menstrual period dates and
actual birthdates were compared. the duration
from last menstrual period to vaginal birth was
noted to be 282 days (median), 281 days (mean)
and 283 days (mode). (34) in a british study involv-
ing 1512 women who had known menstrual dates
and first trimester ultrasound data, where the esti-
mated gestational age discrepancy was within ± 1
day between lmp and ultrasound, the duration of
gestation was estimated for spontaneous births.
the median time to spontaneous birth was 283
days after the lmp (95% ci 282-283 days). (35)
the same study demonstrated that nulliparous
women have a longer gestation compared to mul-
tiparous women. the median time from lmp to
spontaneous delivery was 2 days longer among
nulliparous women compared with multiparous
women (284 vs. 282 days, p < .0001). (35) a longer
gestation for nulliparous women was also shown
in a 1983 retrospective american study of 114
uncomplicated pregnancies. For women having
spontaneous onset of labour, the median gesta-
tion from ovulation for nulliparas was 274 days
and for multiparas it was 269 days. (33) more re-
search is needed in this area.
Determining Date of Conception
basal body temperature has been accepted as an
indicator of the approximate time of ovulation.
the onset of pregnancy can be assessed with rea-
sonable reliability when basal body temperature
recordings are made before and after conception.
(32) if the conception date is known, either by
charting basal body temperature or because con-
ception occurred by insemination or in vitro fer-
tilization, adding the “standard” estimate of 266
days will provide the most accurate EDb. (36,37)
the accurate recording of basal body temperature,
mucus monitoring or urine-test kits can predict
with a higher degree of accuracy when ovulation
actually occurs, compared to the lmp. (37)
Summary Statement
Having a known conception date will provide the
most accurate estimate of EDB. (II-2)
MENSTRUAL DATING
Establishing an EDb using menstrual dating alone
assumes a 28-day cycle, with ovulation occurring
on day 14-15. though this is accurate for many
women, there are times when it is not. Franz carl
naegele was a 19th century obstetrician who pub-
lished an easy method to establish EDb: adding 7
days from the menstrual period and counting back
3 months. naegele’s rule establishes an EDb that is
approximately 280 days from the lmp and is wide-
ly used by health care practitioners.
naegele’s original citation did not make it clear
if the calculations were based on the first or last
day of lmp, however the first day is now standard.
(26,37,38) it has been asserted that naegele’s rule
was never based on empirical data, but rather on
8 Association of Ontario Midwives
observations that women were most likely to con-
ceive just after menstruation, as well as the ac-
cepted normal gestation period for humans of 10
lunar or 9 calendar months. (33,38)
naegele’s rule may result in calculating gesta-
tion periods from 280-283 days depending on
the months in question, due to variations in the
number of days in different months. (30) other
identified problems with naegele’s rule include:
inaccurate recall by women of the date on which
the menstrual period began, variations in the fol-
licular phase of the menstrual cycle and difficulty
in determining whether the last bleeding episode
was a menstrual period or bleeding that may be at-
tributed to breakthrough bleeding or implantation
bleeding, oral contraceptive use, and any factor
that could influence ovulation timing. (31,36,39)
Variations in the Follicular Phase of the Menstrual Cycle / Timing of Ovulation
assigning a 14-day interval between menstruation
and ovulation will be inaccurate for women with
irregular cycles or delayed ovulation. in 2 stud-
ies of a population who have charted their basal
body temperature and where dating by ovula-
tion is possible, 70% of the population classified
as postterm from their lmp using naegele’s rule,
were incorrectly dated. (32,40) this is due to a pro-
longed follicular phase or delayed ovulation and
was demonstrated by basal body temperature and
coital records. Delayed ovulation may involve the
apparent prolongation of pregnancy when ovula-
tion dates are unknown and EDb is calculated us-
ing menstrual dates only. (32)
several studies have shown that the 28-day cycle,
with ovulation on day 14 is not applicable to all
women. in a study of 5688 women, 30% reported
an average cycle length greater than 30 days. (41)
in another study of 498 women with normal men-
strual cycles, there was a range of 7 to 19 days for
the luteal phase, and only 10% of women ovulated
on day 14. (42) a study examining the timing of
ovulation and fertility found only 30% of women
with normal 28-day cycles are ‘fertile’ between
days 10 and 17. (43) since naegele’s rule assumes
that ovulation occurs 14 to 15 days after the first
day of the lmp, adjusting the EDb according to the
woman’s cycle length may increase the accuracy of
the estimation.
Inaccurate Recall of LMP Date
a non-biological factor that reduces the accuracy
of menstrual dating is the inaccurate recall of
menstrual dates or the length between menstrual
cycles. (33,36) one study looking at women’s recall
of the day of their last menstrual period noted sev-
en digits: 1, 5, 10, 15, 20, 25 and 28 to have been re-
ported more frequently than expected. the study
found that women were 2.5 times more likely to
report menstruating on day 15 than any other day,
showing a digit preference. (36,44) the authors at-
tributed this to rounding and surmised that this
would lead to overestimating the gestational age,
thus reducing the accuracy of menstrual dating for
establishing EDb. women who reported the first
day of lmp with non-preferred numbers were like-
ly to have more accurate lmp-based estimations
of gestational age as measured by agreement with
dating ultrasound. (44)
Gestational Wheels
gestational wheel EDb is determined using either
an lmp date or an ultrasound-determined gesta-
tional age. the use of gestational wheels to cal-
culate EDb from lmp is not recommended, as they
are prone to error. this may be due to the poor
quality control in the production of pregnancy
wheels such as the lines not being evenly spaced or
concentrically aligned. the loosening of the cen-
tral mounting of the gestational wheel may also
contribute to errors in calculating EDb. Five-day er-
rors are typical between wheels and they often do
not correlate with naegele’s rule. (30)
Summary Statement
Factors that contribute to inaccurate calculation of
EDB using menstrual dating include: inaccurate re-
call by women of the date on which the menstrual
period began, variations in the follicular phase of
the menstrual cycle and any factor that could influ-
ence ovulation timing. (II-2)
Using a gestational wheel to calculate EDB is not
recommended. Counting 266 days from a known
conception date or 280 days from a certain first day
of LMP is preferable. (II-2)
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 9
MENSTRUAL HISTORY
taking a thorough menstrual history is important
in order to be as accurate as possible in establishing
EDb. midwives should elicit as much menstrual and
fertility information as possible from the woman
to determine an accurate EDb. During menstrual
history taking, the following information should
be discussed: lmp date (asking questions to help
women to recall the date as accurately as possible),
history of previous menstrual cycles, duration and
amount of bleeding during menses, contraception
use and timing of sexual activity. women may also
have knowledge of their potential conception date
if using a fertility awareness method such as chart-
ing basal body temperature, use of an ovulation
predictor kit or if they conceived through assisted
fertility methods. gathering this information will
help to establish the EDb based on lmp in a man-
ner that may more closely reflect a true estimate.
however, it is important to consider that even the
most careful history taking and skilled questioning
may not overcome errors in women’s recollection
of their menstrual dates or variations in their follic-
ular cycle which will contribute to errors in the esti-
mation of EDb. once an EDb has been determined,
corroborate or reassess estimated dates based on
physical assessments which may include fundal
height measurements, timing of quickening and/
or how early fetal heart may be auscultated with
a fetoscope.
ULTRASOUND DATING
some midwifery clients may request the use of ul-
trasound as a routine part of their pregnancy while
others choose to decline. Ultrasound biometry as
it relates to preventing postdates pregnancy in es-
tablishing as accurate as possible an EDb will be
discussed. the risks and benefits, as well as costs
of using ultrasound in the uncomplicated preg-
nancy are beyond the scope of this cpg.
the use of ultrasound for pregnancy dating is
based on the premise that there is very little varia-
tion in the growth rate of the fetus, particularly in
early pregnancy. Knowing the size of the fetus by
ultrasound is thought to be equivalent to know-
ing the gestational age, with a margin of error of
8%. (36,37) it is important to recognize that dating
a pregnancy using the lmp does so by determin-
ing the length of pregnancy, while an ultrasound
estimates dates based on fetal size. a limitation
of using ultrasound is that fetal size references
themselves are based on having a certain lmp as
the original standard, which is not accurate for all
women, as discussed above. globally, a variety of
policies exist for dating a pregnancy when both a
valid lmp and ultrasound date is available: (7, 10
or 14-day rules). (29) currently, the society of ob-
stetricians and gynaecologists of canada recom-
mends changing the EDb if ultrasound estimation
is ± 5 days in the first trimester (t1), or ± 10 days
in the second trimester (t2). these guidelines ap-
proximate the 8% margin of error of ultrasound,
but are less accurate as length of gestation in-
creases in each trimester. (45)
Comparing the Effect of Menstrual Dating vs. Ultrasound Dating
a 2002 study involving 3655 women who had a
known last lmp as well as an early ultrasound (< 21
weeks) examined the precision of different dating
methods to estimate gestational age. they evalu-
ated the accuracy of pregnancy dating by lmp
alone, ultrasound estimates alone and algorithms
that used the lmp date unless there was (a) more
than 7 days’ difference in the EDb when compared
to ultrasound and (b) more than 14 days’ difference
in the EDb when compared to ultrasound. when
using lmp information alone, many more women
had their babies after 41 weeks (12.1%), compared
to their EDb based on ultrasound alone (3.4%) or
when their EDb was adjusted when it differed by
more than 7 days (4.5%) or 14 days (3.5%) from the
lmp estimate. this study confirms the findings of
previous studies that determined that menstrual
dating alone was more likely to overestimate ges-
tational age, by not accounting for delayed ovula-
tion. (36)
another study of 1867 singleton live births com-
pared first trimester report of lmp and first trimes-
ter ultrasound, and examined whether differences
between estimates varied by maternal and infant
characteristics. lmp classified more births as post-
term than ultrasound (4.0% vs. 0.7%). results in-
dicate first trimester report of lmp reasonably
approximates gestational age obtained from first
trimester ultrasound, but the degree of discrep-
ancy between estimates varies by some maternal
characteristics: younger age, ethnicity, high bmi
and low birth weight. more research is needed in
this area to clearly identify subpopulations at high-
er risk for EDb calculation errors, to reduce misdi-
agnoses of postterm pregnancy. (46)
in a retrospective study of 11 510 women with sin-
gleton pregnancies, reliable lmp and delivery after
37 weeks were divided into 4 groups: women who
delivered at term (within 259-295 days) according
to both the ultrasound and the lmp; women who
delivered postterm according to the lmp estimate
but not according to the ultrasound estimate;
women who delivered postterm according to the
ultrasound estimate but not according to the lmp;
and women who delivered postterm according
to both the ultrasound and the lmp estimates.
there was no significant difference in mortality
between the term group and the other 3 study
groups. there was no significant increase in the
risk for apgar score of < 7 after 5 min or transfer
to the neonatal intensive care unit for pregnancies
that were defined as postterm according to the
last menstrual period estimate but not according
to the ultrasound estimate. however, there was
an increased risk for apgar score of < 7 after 5 min
in the group that was postterm according to the
ultrasound estimate but not according to the last
menstrual period estimate (rr 4.96, 95% ci 1.97-
12.5). this suggests that the effect of ultrasound
in changing the EDb to a later date leading to preg-
nancies becoming postterm according to the lmp
estimate but not according to the ultrasound esti-
mate does not have any adverse consequences for
fetal outcome. (47)
a retrospective study of 34 249 singleton pregnan-
cies compared the accuracy of EDb estimations
in predicting the actual date of delivery when us-
ing ultrasound alone, menstrual date alone or an
algorithm where lmp dates were used and only
adjusted if there was a discrepancy with the ul-
trasound dates of 7, 10 or 14 days. women who
had both certain menstrual dates and ultrasound
biometry were included in the study. menstrual
histories were taken by midwives and only entered
if the woman was certain of her dates, her men-
strual cycle had been regular and no oral contra-
ceptives had been used in the previous 3 months.
Delivery took place within ± 7 days of the EDb in
49.5% of the cases if lmp was used and 55.2% of
the cases if ultrasound only was used. the mean
lengths of pregnancy were shortest if dating was
by ultrasonography alone (279.1 days) and longest
for menstrual dating alone (281.8 days). prediction
errors (calculated as estimated gestational age at
delivery - 280) of lmp estimates were larger, and
differed significantly from ultrasound estimates
alone. this longer average length of pregnancy
when using menstrual dates to calculate EDb in-
creases the incidence of women whose pregnancy
lasts beyond 41 weeks. whether postterm is de-
fined as being 41, 41+3 or 42 weeks, in this study,
ultrasonography alone resulted in lower numbers
being classified as postterm as did the other dating
methods (see table 2). because induction is recom-
mended in many communities when women are
postterm, the dating method used will affect the
number of cases that fall into this category. the
authors predict that if ultrasound prior to 20 weeks
is used to calculate the EDb, it will reduce the num-
ber of pregnancies that last beyond 42 weeks by
70%. (29)
a canadian study of 44 623 women in a tertiary
hospital that included all live or stillborn infants in-
cluding multiple births compared lmp with ultra-
sound at 16 to 18 weeks. six methods of EDb cal-
culation were tested: lmp alone, lmp if ultrasound
was ± 14 days, lmp if ultrasound was ± 10 days,
lmp if ultrasound was ± 7 days, lmp if ultrasound
was ± 3 days, and ultrasound alone. concordance
between lmp and ultrasound was within 3 days for
46.6% of all births, and 90.7% were within 14 days.
the proportion of births greater than or equal to
42 weeks was 6.4% for lmp alone and 1.9% for ul-
trasound alone. births greater than or equal to 41
weeks decreased by nearly 50% with use of early
ultrasound vs. lmp estimates. (48)
Comparing the Accuracy of First Trimester and Second Trimester Ultrasound Dating
there is some evidence that first trimester ultra-
sound dating is more effective at preventing in-
10 Association of Ontario Midwives
Table 2: Percentage of cases requiring induction of labour for postdates according to induction policy and gestational dating method. From: (29)
Postdates management policy (day of induction)
Dating method 287 (41 weeks) 290 (41+3 weeks) 294 (42 weeks)
Scan only 19.2% 11.5% 3.5%
7-day rule 21.6% 12.7% 4.5%
10-day rule 23.4% 14.0% 5.3%
14-day rule 25.2% 15.6% 6.5%
LMP only 29.6% 20.3% 11.5%
duction of labour than second trimester dating.
in a study of 218 women who were randomly al-
located to a first or second trimester ultrasound for
the purpose of dating the pregnancy, the EDb was
adjusted if the difference was more than 5 days
between the date calculated by the last menstrual
period (lmp) and first trimester ultrasound dates,
or for a difference of more than 10 days with a
second trimester ultrasound. of women assigned
to the first trimester screening group, 41.3% had
their gestational age adjusted on the basis of the
crown-rump length measurement. of 92 women
randomly assigned to the second trimester screen-
ing group, 10.9% were corrected as a result of bi-
ometry (rr 0.26, 95% ci 0.15-0.46, p < .001). Fewer
women in the first trimester screening group had
labor induced for postterm pregnancy (rr 0.37,
95% ci 0.14-0.96, p = .04). there was a significant
difference in the prevalence of postterm pregnan-
cy between groups. in the first trimester screening
group 6.7% delivered at a gestational age of 287
days or greater, compared with 16.3% in the sec-
ond trimester screening group (rr 0.41, 95% ci
0.18-0.94, p = .03). there were no significant differ-
ences in caesarean section rates, or neonatal out-
comes observed between the two groups. (31)
a cochrane review that included 6 randomized
controlled trials (rcts), with pooled results of 24
195 women, found that routine early ultrasound
in pregnancy was associated with reduced rates
of induction of labour for postterm pregnancy (or
0.61, 95% ci 0.52-0.72). (49)
overall, the use of ultrasound to calculate EDb has
correlated with a greater population level decline in
postterm births than by using lmp. a large ameri-
can study reviewed 42 689 603 natality Data Files
on singleton live births between 22 and 44 weeks’
gestation. the authors compared lmp dates and
ultrasound (t1 or t2). the decline in the american
postterm birth rate from 1990-2002 was 36.6% us-
ing the lmp, and 73.8% using the ultrasound esti-
mate of gestational age. (50) (see Figure 2)
Summary Statement
Ultrasound dating measures the size of the fetus,
which is believed to be equivalent to knowing ges-
tational age, with a margin of error of 8%. LMP dat-
ing estimates the length of the pregnancy. Ultra-
sound dating does not prevent postterm pregnancy;
rather it measures fetal size, addressing errors that
may occur due to LMP dating such as inaccurate re-
call of menstrual dates and factors that influence
ovulation timing. Studies have consistently shown
that the use of ultrasound dates alone result in few-
er postterm births than LMP alone, or any algorithm
used to adjust EDB based on a combination of LMP
and ultrasound estimates. (II-2)
Recommendations
Inform clients that when EDB information 1.
is available from both LMP and ultrasound
measurements, an EDB based on ultrasound
dating prior to 24 weeks is less likely to result
in a postterm pregnancy. (II-2-B)
For women who choose not to have ultra-2.
sound, taking as accurate a menstrual history
as possible is recommended to give a more
precise estimate of pregnancy length. Obtain
as much menstrual and fertility information
as possible from the woman. Corroborate or
reassess estimated dates based on physical
assessments. (III-A)
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 11
INTERVENTIONS USED TO PROMOTE SPONTANEOUS LAbOUR
there are various methods used later in pregnancy
to avoid the need for a postterm induction includ-
ing: sweeping the membranes, evening primrose
oil, homeopathic remedies and acupuncture. these
methods are included in this document because
they are believed to support the natural changes
at the end of pregnancy, rather than to initiate la-
bour. alternative therapies used for the induction
of labour such as castor oil, herbal remedies and
breast stimulation will be considered in a clinical
practice guideline on methods of labour induc-
tion. some of the methods suggested below may
be self-administered, while some require the aid of
a health care practitioner. some are supported by
research and others by anecdotal evidence, physi-
ologic rationales or beliefs of efficacy.
there are many reasons a woman may prefer to
hasten the onset of their labour using home rem-
edies or alternatives to medical induction. west-
fall and benoit conducted a qualitative study to
explore women’s views of prolonged pregnancy
and how they felt about managing the end of the
pregnancy proactively or “letting nature take its
course.” twenty-seven women in british columbia
were interviewed in their third trimester of preg-
nancy and 23 were re-interviewed postpartum.
many of the women favoured a watch-and-wait
approach when interviewed in their third trimes-
ter. however, in the postpartum interview, 9 of the
10 women whose pregnancies lasted beyond 40
weeks’ gestation reported using do-it-yourself pro-
active measures to hasten labour onset and none
of the women requested medical induction. benoit
and westfall concluded that home remedies to
hasten labour allowed women to “guide their own
care rather than follow their caregiver’s order” and
were seen as a way of exercising agency, and re-
sisting loss of control over the childbearing experi-
ence. (51) Use of home remedies may also reflect
the anxiety reported by some women with waiting
for the onset of labour past the EDb.
Sweeping the Membranes
sweeping the membranes, sometimes referred to
as a “stretch and sweep” appears to be an effective
method for reducing the incidence of postterm
pregnancy and the need for induction. this inter-
vention may be particularly helpful if the woman
12 Association of Ontario Midwives
Figure 2: Temporal trend in postterm birth 42 to 44 weeks based on gestation age using menstrual dates and ultrasound estimates of gestational age, US singleton live births, 1990-2002. (Straight line = LMP, dotted line = ultrasound estimate) (50)
0
0
4
6
8
10
12
14
1990 1992 1994 1996 1998 2000 2002
year of birth
post
term
bir
th (4
2+ w
eeks
rat
e (%
)
Table 3: Summary of RCTs of Sweeping Membranes (I-A)
Study Rate of postdates
pregnancy
Induction rate
Sweeping Control Sweeping Control
De miranda
(55)
23% 41% n/a n/a
Dare (57) 3% 16% n/a n/a
berghella
(52)
5.5% 21.7% n/a n/a
cammu (54) 19% 33% 11% 26%
gupta (56) 4% 34% 2% 32%
magann (53) n/a n/a 17% 69%
is nulliparous (52) or has an unfavourable cervix.
(53) the regimens used in 6 different randomized
trials varied from every other day starting at 41
weeks, to weekly starting at 38 weeks, to one sin-
gle event (see table 3 for summary of results). be-
ginning weekly membrane sweeping at 38 weeks
was shown to decrease the number of women
who reached 41+ weeks’ gestation. (52,54) Daily
or every other day membrane sweeping begin-
ning at 41 weeks decreased the number of women
who reached 42 weeks. (53,55) Even a single ses-
sion of membrane sweeping was associated with
an earlier spontaneous labour and reduced need
for induction. (56,57) while the procedure can be
uncomfortable, one study reported that 88% of
women would choose to have it done again in a
subsequent pregnancy. (55) a cochrane review (22
trials) updated in 2009 reported a reduced duration
of pregnancy, a reduced frequency of pregnancies
beyond 41 weeks (rr 0.50) and beyond 42 weeks
(rr 0.28). (58)
Recommendation
Offer sweeping of membranes, when appro-3.
priate, beginning between 38 and 41 weeks,
to reduce the rate of postterm pregnancy and
the need for induction. (I-A)
Evening primrose oil
there are no prospective trials on use of oral eve-
ning primrose oil for cervical ripening. one study
found no benefit to use of oral evening primrose
oil, however, the study was a retrospective chart re-
view that did not report on potential confounding
variables such as the indication for using evening
primrose oil or bishop score. (59) women choosing
to use evening primrose oil may have had one of the
following: unfavorable cervix, history of postdates
pregnancy or medical indication to induce labour.
there is an individual case report linking evening
primrose oil in late pregnancy to inhibited platelet
function in the neonate. (60) no studies were found
on the use of vaginal evening primrose oil.
Acupuncture
acupuncture is believed to stimulate hormonal
changes or the nervous system, eventually stimu-
lating the uterus. three small studies were in-
cluded (212 women) in a cochrane review. the
reviewer found that fewer women using acupunc-
ture required the use of induction methods (rr
1.45, 95% ci 1.08-1.95) compared with standard
care. these studies lacked statistical power and
details on primary outcomes and identified a need
for further study related to the use of acupuncture.
however, the reviewers concluded that acupunc-
ture appears to be safe in late pregnancy and has
no known teratogenic effects. (61) more research
on the risks and benefits of acupuncture is needed
in order to make a recommendation.
Homeopathy
there is insufficient data on the use of homeo-
pathic remedies such as caulophyllum to recom-
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 13
mend their use for the prevention of postterm
pregnancy. one systematic review identified two
trials but the trials had small sample sizes and in-
sufficient detail to allow a recommendation. the
review noted, however, that serious side effects
from homeopathy use are rare and “remedies rec-
ommended for use in pregnancy are not thought
to cause any problems in pregnancy.” (62)
Summary Statement
No recommendations on either using or not using
evening primrose oil, acupuncture or homeopathy
can be made due to the absence of good quality re-
search and subsequent lack of evidence regarding
efficacy. These approaches may be offered as part
of a range of alternatives, including conventional
therapies, discussing the risks and benefits of each
as well as any research gaps.
MANAGEMENT OF POSTDATES PREGNANCY
Background
as clients reach and pass their due dates, decisions
about whether or not to intervene in the postdate
pregnancy have to be made. is intervention neces-
sary? (63) is the prevention of postterm pregnancy
of benefit to the fetus? Do potential benefits to
the fetus outweigh potential risks for the mother?
how can we determine which fetus is at risk of
the rare complications associated with postdates
pregnancy?
these questions are being asked in the context of
increasing induction rates (64,65) and caesarean
section rates, (66) a trend many believe is influ-
enced by medico-legal considerations. (63) in the
Us, between 1990 and 1998, the rate of labour in-
duction increased from 9.5% to 19.4%, though the
indications for the inductions varied. (65,67) this
increase has been attributed to the “availability of
cervical ripening agents, pressure from patients,
conveniences to physicians and litigious con-
straints.” (65) in ontario, the induction rate was
24.7% in 2007/08, with postdates accounting for
32.7% of the total inductions. (68) Emotional stress
for some midwifery clients as their pregnancy be-
comes prolonged, along with an obstetric commu-
nity standard where induction of labour in the un-
complicated pregnancy is offered and encouraged
at 41 weeks have increasingly become obstacles to
the expectant approach to management of post-
dates pregnancy.
the question of whether or not the risks of in-
duction outweigh potential risks to the fetus in
the uncomplicated postdates pregnancy have
been the subject of numerous studies and meta-
analyses yielding conflicting results.
(18,20,21,64,69-72) in their 2009 meta-analysis,
wennerhold et al. describe 3 options in the man-
agement of the prolonged uncomplicated preg-
nancy:
i. a policy of routine induction of labour at a
specified gestational age
ii. Fetal assessment in the prolonged preg-
nancy with intervention based on evidence of fetal
compromise (in practice approach i and ii are often
combined)
iii. no intervention (64)
weighing the risks and benefits of the 3 approach-
es to postdates management are part of what
midwives discuss with their clients. a postdates
discussion should include the potential risks to the
fetus of postterm pregnancy and the risks of la-
bour induction (see table 4) to the fetus and to the
mother. helping clients to interpret this research
is difficult as the research related to the manage-
ment of the uncomplicated postdates pregnancy
is often conflicting, of varying quality and com-
plicated by differences in methods used to date
pregnancies and different protocols used to induce
labour.
COMPARING INDUCTION OF LAbOUR AND EXPECTANT MANAGEMENT
a large retrospective review of births occurring be-
tween 41 and 45 weeks’ gestation (408 631 births)
found that the number of inductions needed to
prevent one perinatal death decreased constant-
ly after 41+0 weeks (nnt: 527 at 41 wks, 195 at
43 wks). (21) in this study the nnt at 42 was not
specified, nor was data available to make this cal-
14 Association of Ontario Midwives
Table 4: Summary of Risks of Induction of Labour (73)
Fetal Risks Maternal Risks
Fetal compromise as a result of
uterine hyperstimulation
For primiparous women:
complications of prolonged
labour or failed induction (e.g.
chorioamnionitis, operative delivery)
neonatal immaturity if dating is
inaccurate
For multiparous women (p > 3):
uterine hyperstimulation
Fetal compromise as a result of
prolonged labour
increased use of epidural analgesia
(72)
culation. however, in another review of morbid-
ity and mortality rates of conservatively managed
postterm pregnancies in a norway hospital during
the period from 1989-1999, an nnt of 370 at 42
weeks was calculated. (16)
one retrospective review of 3262 women at or
past their due dates found that induction was as-
sociated with a 17% increase in epidurals and a 5%
increase in cs. (72)
a case control study compared 360 women in-
duced at 42 weeks with 486 controls who were
managed expectantly with serial fetal monitoring.
the induction group had a higher operative deliv-
ery rate (or 1.46, 95% ci 1.34-2.01). (74) research-
ers have pointed out the challenge of retrospective
studies with regards to their potential for bias (74)
and have called for larger randomized studies be-
fore changing policy regarding the management
of postterm pregnancy. (75)
randomized studies and systematic reviews have
resulted in conflicting conclusions. management of
postdates pregnancy was significantly influenced
in canada by the canadian multicenter post-term
pregnancy trial (cmppt) published in 1992. this
trial randomized a total of 3 407 women between
41 and 44 weeks’ gestation into an induction
group (1701) and antenatal monitoring/expectant
management group (1706). (18) the study found
similar rates of perinatal mortality and neonatal
morbidity in the 2 groups but a higher rate of cs
in the expectant management group (24.5% vs.
21.2%, p = .03). two limitations of the study ac-
knowledged by the authors were the lack of blind-
ing and the different methods of induction. in the
induction group, intracervical prostaglandin gel
was used when the cervix was less than 3 cm di-
lated, whereas the expectant management group
was induced with oxytocin alone which could have
affected the success of the induction process. the
clinicians providing intrapartum care were not
blinded to the women’s group allocation, possibly
influencing the clinician’s threshold for interven-
ing and performing a cs. subsequent commen-
tary on the validity of the cmppt has noted that
the higher rate of cs in the expectant group can
be almost completely accounted for by more op-
erations for fetal distress (8.3% vs. 5.7%) and sug-
gested that clinicians were more likely to respond
to fetal tracings among those in the expectant
management group. (76) lack of blinding could
also have had an impact on the results of a case
control study by luckas et al. where the rate of cs
(rr 1.9) and nicU admissions (rr 2.69) were higher
for the postdates pregnancy group while the inci-
dence of low apgars and neonatal pathology were
the same. the authors conclude “a lower threshold
for clinical intervention in pregnancies perceived
to be ‘at-risk’ may be a significant contributing fac-
tor.” (77) two smaller rcts (440 and 508 women
respectively) did not find a significant difference
in neonatal mortality, morbidity or cs rate when
comparing routine induction with expectant man-
agement. (19)
a recent canadian study from winnipeg attempted
to validate the findings of the cmppt at a tertiary
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 15
care hospital. the study examined the outcomes of
1367 women in non-randomized contemporane-
ous cohorts of nulliparous women at 41+0 weeks
who had planned either expectant management
or induction. when grouped by intention to treat,
caesarean rates were not significantly different at
17.7% for expectant management and 21.3% in-
duction respectively, (p = .09). in an analysis that
compared all spontaneous labours (regardless of
whether they planned expectant management or
induction of labour) with a group where induction
of labour was planned and carried out, caesarean
rates were 16.6% (spontaneous labour group) vs.
25.4% (induction of labour group), (p = .001). the
authors conclude that the results of the cmppt
may not be valid for similar hospital environments
with low cs rates and strict indications for induc-
tion. (71)
a cochrane review comparing induction at 41
weeks with at least one additional week of expect-
ant management found no significant difference
in the cs rate. there was a significant decrease in
mas in the induction group (rr 0.29). the review
did report a lower perinatal mortality rate in the
41-week induction group (0.03% vs. 0.33%), even
after all deaths due to congenital abnormalities
were excluded (0 vs. 0.21%) but this finding was
not statistically significant. (70) a 2009 systemic
review by wennerholm et al. found that expectant
management was not associated with a higher risk
of perinatal mortality but was associated with an
increased risk of mas (rr 0.43, 95% ci 0.23-0.79).
the expectant management group had more cae-
sarean deliveries but when the hannah trial was
excluded in a sensitivity analysis, this difference
was no longer significant. in addition, the authors
assessed all of the 13 trials included to be of poor
to fair quality and inadequately powered to detect
a rare outcome such as perinatal mortality. (64)
(see table 5)
there is some evidence that the decision to induce
with postdates pregnancy should take into consid-
eration estimated fetal weight: small for gestation-
al age babies appear to be at higher risk postdates.
one small, retrospective analysis of 143 Japanese
women with an uncomplicated pregnancy induced
at 42 weeks found that primiparous women with
babies over 3600 g compared with women whose
babies weighed less than 3600 g had a lower rate
of caesarean section for fetal distress during la-
bour (1/5 vs. 14/18, p < .05) but a higher risk of in-
duction failure due to an unfavourable cervix. the
group with babies weighing < 3600 g were also
statistically more likely to have a lower umbilical
artery blood ph (ph < 7.20) than the group with
babies weighing > 3600 g (14/58 vs. 0/22, p < .01).
in multiparous women there was no significant
difference in obstetrical outcomes between wom-
en with babies over and under 3600 g. it should be
noted that the study population was very small
and results may not be generalizable. (78)
Summary Statement
While there is some evidence that increasing gesta-
tional age is associated with a higher rate of peri-
natal complications, clinical research has not estab-
lished the optimal gestational age to induce labour
in order to avoid adverse outcomes. (I) Perinatal risk
seems to be higher for postdates babies who are also
small for gestational age. (II-2) For AGA babies, a
policy of expectant management until 42+0 weeks’
gestation has the potential advantage of reducing
rates of induction (I) and epidural (II-2).
Recommendations
Prior to 41+0 weeks’ gestation, discuss the 4.
risks and benefits of induction of labour be-
tween 41 and 42 weeks’ gestation and offer
induction by 42+0 weeks’ gestation. (II-2-A)
Inform clients that the absolute risk of peri-5.
natal death from 40+0 weeks to 41+0 weeks
to 42+0 weeks’ gestational age changes from
2.72/1000 to 1.18/1000 to 5.23/1000; cur-
rently available research is not of high quality
and has not established an optimal time for
induction. Therefore, women with uncom-
plicated postdates pregnancies should be
offered full support in choices that will allow
them to enter spontaneous labour. A policy
of expectant management to 42+0 weeks
following an informed choice discussion is
the most appropriate strategy for women
who wish to maximize their chance of normal
birth. (II-2-A)
16 Association of Ontario Midwives
Table 5: Comparison of Expectant Management (EM) to Induction of Labour (IOL) at 41 weeks
Comparison of expectant management to induction of labour at 41 weeks
Study CS rate: Intention to Treat
EM IOL
hannah 1992 (18)
rct, n = 3407
24.5% 21.2%
luckas 1998 (77)
cohort study
rr 1.9; 95%ci
1.29-2.85
n/a
heimstad 2007 (20)
rct, n = 508
no significant difference
heimstad 2008 (21)
retrospective review,
n = 98 559
nnt = 527
(to prevent 1 perinatal
death)
gulmezoglu 2009 (70)
cochrane review
19 trials, n = 7984
no significant difference
pavicic 2009 (71)
cohort study, n = 1367
17.7% 21.3%; p = .09
Duff, 2000 (72)
retrospective review,
n = 3262
n/a 5% increase
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 17
For women choosing expectant management 6.
beyond 42+0 weeks, discuss the lack of clear
evidence on which to base a recommenda-
tion regarding expectant management other
than a trend towards increasing perinatal
morbidity and mortality with increasing ges-
tational age (II-2-A)
FETAL SURVEILLANCE FOR PREGNANCIES AT 41+0 WEEKS AND bEYOND
none of the studies reviewed have validated an
optimal starting time or frequency for fetal sur-
veillance at and beyond term. the most commonly
used methods for postdates fetal surveillance are
the non-stress test, amniotic fluid index, biophysi-
cal profile and fetal movement counting. no evi-
dence was found to show that a non-stress test is
an effective way to monitor postdates pregnancies
due to its low sensitivity and low positive predic-
tive value (less than 50%). (79) low amniotic fluid
index (< 5 cm) is associated with adverse perinatal
outcomes but on its own has a low sensitivity (11%
to 28%) for the prediction of morbidity. (80-82)
one study found the biophysical profile to have
high specificity and high negative predictive value;
(83) however, another study found that, while a
modified bpp did result in more abnormal find-
ings, there was not an improvement in neonatal
outcomes when compared to aFi alone. (83)
while maternal awareness of fetal movement is
associated with good outcomes, no specific meth-
od for fetal movement counting has been shown
to be beneficial in reducing perinatal mortality or
morbidity in low-risk pregnancies. (84-86) while
no studies tested or compared specific schedules
for postdates fetal surveillance, the five rcts re-
viewed all had very low perinatal mortality and
morbidity rates and their respective surveillance
protocols are summarized in table 6.
18 Association of Ontario Midwives
Summary Statement
Although non-stress tests and formal fetal move-
ment counting are commonly used as monitoring
strategies there is very little evidence to demonstrate
their efficacy. (II-3-C)
The efficacy of other methods such as amniotic fluid
index (AFI) and bio-physical profile (BPP) are sup-
ported by limited evidence. (II-2)
Recommendation
For women choosing expectant management 7.
of pregnancy at and beyond 41+0 weeks’ ges-
tation, offer ultrasound twice weekly, starting
between 41 and 42 weeks and continuing
until delivery to assess fetal well-being and
amniotic fluid volume. (II-2-A)
CONCLUSION
management of the uncomplicated pregnancy be-
yond 41+0 weeks occurs commonly in midwifery
practice. though overwhelmingly these babies will
be born healthy and without complications, there
is research evidence indicating there are increased
risks associated with increasing gestational age.
not all regions have equal or equitable access to
ultrasound for dating or antenatal fetal monitor-
ing. midwives, along with their clients, should de-
termine the best available methods for estimating
gestational age. midwives should also determine
the best methods for fetal surveillance in their com-
munities during expectant management past 41+0
weeks, determined by access to technologies and
women’s risk tolerance.
the management of postdates pregnancy, and the
decision whether or not elevated risk may warrant
induction of labour is an ongoing debate among
the obstetric community. the evidence available to
date about the comparison of expectant manage-
ment to induction of labour for postdates pregnan-
cy is conflicting and not easily comparable due to
different study protocols and inadequate study size
to detect rare outcomes.
since the outcomes of interest in the management
of prolonged pregnancy occur very infrequently,
very large numbers of study participants are re-
quired to achieve adequate statistical power and
provide convincing evidence for either expectant
management or induction of labour. Until this evi-
dence becomes available, midwives should discuss
these areas of clinical uncertainty with pregnant
women in the spirit of informed choice. Expectant
management until 42+0 weeks can be expected to
support normal birth, along with the associated
benefits of a labour occurring through the mother’s
own efforts. in the absence of clear evidence and
following informed choice discussions about risks
and benefits of both strategies women themselves
are best suited to make decisions in the absence of
clear evidence based on their own risk tolerance
and unique circumstances.
Table 6: Fetal Surveillance Protocols Used In Postdates Trials
Trial: Starting week: Fetal surveillance protocol:
hannah 1992
(18)
41 wks Daily kick counts, non-stress test (nst)
3x/wk, U/s for aFV 2-3x/wk until 44
wks
nichhDn 1994
(19)
41 wks nst and U/s for aFV 2x/wk until 44 wks
chanrachakul
2003 (87)
41+3 wks nst and aFi once/wk until 43 wks then
2x/wk until 44 wks
roach 1997 (88) 42 wks nst and aFi 2x/wk
heimstad 2007
(20)
41+2 wks U/s for EFw and aFi, nst every third
day until 42+6 wks
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 19
RISK MANAGEMENT
practice groups may wish to create a written pro-
tocol specific to the practice group that documents
which of the recommendations within the clinical
practice guideline they are adopting and how they
are putting into practice those recommendations,
including what would be included in an informed
choice discussion with each client. midwives are
advised to document clearly that an informed
choice discussion has taken place. if the practice
group has a written protocol about what should be
discussed with each client, that discussion should
be followed. any deviation from that discussion
should also be documented in the woman’s chart.
if there is no protocol about what information is
provided then documentation in the woman’s
chart should provide details of that discussion. if,
based on the client’s health or risk status, the mid-
wife makes recommendations for surveillance or
intervention that the client declines, the midwife
should document that her recommendation was
declined.
ACKNOWLEDGEMENTS
the association of ontario midwives acknowledg-
es the support of the ontario ministry of health
and long-term care and ryerson University in
providing resources for the development of this
guideline.
SUMMARY OF RECOMMENDATIONS
Inform clients that when EDB information 1.
is available from both LMP and ultrasound
measurements, an EDB based on ultrasound
dating prior to 24 weeks is less likely to result
in a postterm pregnancy. (II-2-B)
For women who choose not to have ultra-2.
sound, taking as accurate a menstrual history
as possible is recommended to give a more
precise estimate of pregnancy length. Obtain
as much menstrual and fertility information
as possible from the woman. Corroborate or
reassess estimated dates based on physical
assessments. (III-A)
Offer sweeping of membranes, when appro-3.
priate, beginning between 38 and 41 weeks,
to reduce the rate of postterm pregnancy and
the need for induction. (I-A)
Prior to 41+0 weeks’ gestation, discuss the 4.
risks and benefits of induction of labour be-
tween 41 and 42 weeks’ gestation and offer
induction by 42 weeks’ gestation. (II-2-A)
Inform clients that the absolute risk of peri-5.
natal death from 40+0 weeks to 41+0 weeks
to 42+0 weeks’ gestational age changes from
2.72/1000 to 1.18/1000 to 5.23/1000; cur-
rently available research is not of high quality
and has not established an optimal time for
induction. Therefore, women with uncom-
plicated postdates pregnancies should be
offered full support in choices that will allow
them to enter spontaneous labour. A policy
of expectant management to 42+0 weeks
following an informed choice discussion is
the most appropriate strategy for women
who wish to maximize their chance of normal
birth. (II-2-A)
For women choosing expectant management 6.
beyond 42+0 weeks, discuss the lack of clear
evidence on which to base a recommendation
regarding expectant management other than
a trend towards increasing perinatal morbid-
ity and mortality with increasing gestational
age (II-2-A)
For women choosing expectant management 7.
of pregnancy at and beyond 41+0 weeks’ ges-
tation, offer ultrasound twice weekly, start-
ing between 41 and 42 weeks and continuing
until delivery to assess fetal well-being and
amniotic fluid volume. (II-2-A)
REFERENCES(1) canadian task Force on preventive health
care. new grades for recommendations from the canadian task Force on preventive health care. cmaJ 2003 aug 5;169(3):207-208.
(2) the agrEE collaboration. appraisal of guide-lines for research & Evaluation (agrEE) instrument. 2001.
(3) association of ontario midwives. collated response: a Values based approach to cpg Development. 2006.
(4) mogren i., stenlund h., hogberg U. recur-rence of prolonged pregnancy. int J Epidem 1999;28:253-257.
(5) Enkin m., Keirse m., neilson J., crowther c., Duley l., hodnett E., hofmey J. prelabour rupture of the membranes. a guide to effec-tive care in pregnancy and childbirth. 3rd ed. ed. oxford: oxford University press; 2000.
(6) Davies r. i’m ready for you baby. why won’t you come? Further discussion about post-dates pregnancy and the intervention of induction of labour. nZcom J 2005 october 2005(33).
(7) treger m, hallak m, silberstein t, Friger m, Katz m, mazor m. post-term pregnancy: should induction of labor be considered before 42 weeks?. J.matern.Fetal.neonatal med. 2002 Jan;11(1):50-53.
(8) Denison Fc, price J, graham c, wild s, liston wa. maternal obesity, length of gestation, risk of postdates pregnancy and spontane-ous onset of labour at term. bJog 2008 may;115(6):720-725.
(9) hovi m, raatikainen K, heiskanen n, heinonen s. obstetric outcome in post-term preg-nancies: time for reappraisal in clinical management. acta obstet.gynecol.scand. 2006;85(7):805-809.
(10) sue-a-Quan aK, hannah mE, cohen mm, Foster ga, liston rm. Effect of labour induc-tion on rates of stillbirth and cesarean sec-tion in post-term pregnancies. cmaJ 1999 apr 20;160(8):1145-1149.
(11) olesen aw, westergaard Jg, olsen J. prena-tal risk indicators of a prolonged pregnancy. the Danish birth cohort 1998-2001. acta obstet.gynecol.scand. 2006;85(11):1338-1341.
(12) Kitlinski laczna m, Kallen K, marsal K, olofs-son p. skewed fetal gender distribution in prolonged pregnancy: a fallacy with conse-quences. Ultrasound obstet.gynecol. 2003 mar;21(3):262-266.
(13) Divon my, Ferber a, nisell h, westgren m. male gender predisposes to prolongation of pregnancy. am.J.obstet.gynecol. 2002 oct;187(4):1081-1083.
(14) caughey ab, stotland nE, washington aE, Escobar gJ. who is at risk for prolonged and postterm pregnancy? am.J.obstet.gynecol. 2009 Jun;200(6):683.e1-683.e5.
(15) Kistka Za, palomar l, boslaugh sE, De-
baun mr, DeFranco Ea, muglia lJ. risk for postterm delivery after previous post-term delivery. am.J.obstet.gynecol. 2007 mar;196(3):241.e1-241.e6.
(16) nakling J, backe b. pregnancy risk increases from 41 weeks of gestation. acta obstet.gynecol.scand. 2006;85(6):663-668.
(17) caughey ab, stotland nE, washington aE, Escobar gJ. maternal and obstetric complications of pregnancy are associated with increasing gestational age at term. am.J.obstet.gynecol. 2007 Feb;196(2):155.e1-155.e6.
(18) hannah mE, hannah wJ, hellmann J, hewson s, milner r, willan a. induction of labor as compared with serial antenatal monitoring in post-term pregnancy. a ran-domized controlled trial. the canadian mul-ticenter post-term pregnancy trial group. n.Engl.J.med. 1992 Jun 11;326(24):1587-1592.
(19) a clinical trial of induction of labor versus expectant management in postterm preg-nancy. the national institute of child health and human Development network of maternal-Fetal medicine Units. am.J.obstet.gynecol. 1994 mar;170(3):716-723.
(20) heimstad r, skogvoll E, mattsson la, Johan-sen oJ, Eik-nes sh, salvesen Ka. induction of labor or serial antenatal fetal monitor-ing in postterm pregnancy: a randomized controlled trial. obstet.gynecol. 2007 mar;109(3):609-617.
(21) heimstad r, romundstad pr, salvesen Ka. induction of labour for post-term pregnancy and risk estimates for intrauterine and perinatal death. acta obstet.gynecol.scand. 2008;87(2):247-249.
(22) caughey ab, washington aE, laros rK,Jr. neonatal complications of term pregnancy: rates by gestational age increase in a con-tinuous, not threshold, fashion. am.J.obstet.gynecol. 2005 Jan;192(1):185-190.
(23) heimstad r, romundstad pr, Eik-nes sh, sal-vesen Ka. outcomes of pregnancy beyond 37 weeks of gestation. obstet.gynecol. 2006 sep;108(3 pt 1):500-508.
(24) hilder l, costeloe K, thilaganathan b. prolonged pregnancy: evaluating gestation-specific risks of fetal and infant mortality. br.J.obstet.gynaecol. 1998 Feb;105(2):169-173.
(25) world health organization. health status statistics: mortality. 2010; available at: http://www.who.int/healthinfo/statistics/indneonatalmortality/en/. accessed June 23, 2010, 2010.
(26) cunningham F, leveno K, bloom s, hauth J, gilstrap l, wenstrom K editors. williams obstetrics. 22nd ed. new york: mcgraw-hill; 2005.
(27) clausson b, cnattingius s, axelsson o. out-comes of post-term births: the role of fetal growth restriction and malformations. ob-stet.gynecol. 1999 nov;94(5 pt 1):758-762.
(28) sylvestre g, Fisher m, westgren m, Divon my. non-reassuring fetal status in the prolonged pregnancy: the impact of fetal weight. Ultrasound obstet.gynecol. 2001 sep;18(3):244-247.
(29) mongelli m, wilcox m, gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. am.J.obstet.gynecol. 1996 Jan;174(1 pt 1):278-281.
(30) ross mg. circle of time: errors in the use of the pregnancy wheel. J.matern.Fetal.neona-tal med. 2003 Dec;14(6):370-372.
(31) bennett Ka, crane Jm, o’shea p, lacelle J, hutchens D, copel Ja. First trimester ultrasound screening is effective in reducing postterm labor induction rates: a random-ized controlled trial. am.J.obstet.gynecol. 2004 apr;190(4):1077-1081.
(32) saito m, yazawa K, hashiguchi a, Kumasaka t, nishi n, Kato K. time of ovulation and prolonged pregnancy. am.J.obstet.gynecol. 1972 Jan 1;112(1):31-38.
(33) mittendorf r, williams ma, berkey cs, cotter pF. the length of uncomplicated human ges-tation. obstet.gynecol. 1990 Jun;75(6):929-932.
(34) bergsjo p, Denman Dw,3rd, hoffman hJ, meirik o. Duration of human singleton pregnancy. a population-based study. acta obstet.gynecol.scand. 1990;69(3):197-207.
(35) smith gcs. Use of time to event analysis to estimate the normal duration of hu-man pregnancy. hum.reprod. 2001 July 1;16(7):1497-1500.
(36) savitz Da, terry Jw, Dole n, thorp Jm, siega-riz am, herring ah. comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. am.J.obstet.gynecol. 2002 12;187(6):1660-1666.
(37) hunter la. issues in pregnancy dating: re-visiting the evidence. J.midwifery womens health 2009 may-Jun;54(3):184-190.
(38) baskett t, nagele F. naegele’s rule: a reap-praisal. bJog 2000;107:1433-1435.
(39) nichols cw. postdate pregnancy. part ii. clinical implications. J.nurse.midwifery 1985 sep-oct;30(5):259-268.
(40) boyce a, mayaux mJ, schwartz D. clas-sical and “true” gestational postma-turity. am.J.obstet.gynecol. 1976 aug 1;125(7):911-914.
(41) berg at. menstrual cycle length and the cal-culation of gestational age. am.J.Epidemiol. 1991 mar 15;133(6):585-589.
(42) baird DD, mcconnaughey Dr, weinberg cr, musey pi, collins Dc, Kesner Js, et al. application of a method for estimating day of ovulation using urinary estrogen and pro-gesterone metabolites. Epidemiology 1995 sep;6(5):547-550.
(43) wilcox aJ, Dunson D, baird DD. the timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective
20 Association of Ontario Midwives
study. bmJ 2000 nov 18;321(7271):1259-1262.
(44) waller DK, spears wD, gu y, cunningham gc. assessing number-specific error in the recall of onset of last menstrual period. pae-diatr.perinat.Epidemiol. 2000 Jul;14(3):263-267.
(45) clinical practice obstetrics committee, ma-ternal Fetal medicine committee, Delaney m, roggensack a, leduc Dc, ballermann c, et al. guidelines for the management of pregnancy at 41+0 to 42+0 weeks. J.obstet.gynaecol.can. 2008 sep;30(9):800-823.
(46) hoffman cs, messer lc, mendola p, savitz Da, herring ah, hartmann KE. comparison of gestational age at birth based on last menstrual period and ultrasound during the first trimester. paediatr.perinat.Epidemiol. 2008 nov;22(6):587-596.
(47) tunon K, Eik-nes sh, grottum p. Fetal out-come in pregnancies defined as post-term according to the last menstrual period esti-mate, but not according to the ultrasound estimate. Ultrasound obstet.gynecol. 1999 Jul;14(1):12-16.
(48) blondel b, morin i, platt rw, Kramer ms, Usher r, breart g. algorithms for combin-ing menstrual and ultrasound estimates of gestational age: consequences for rates of preterm and postterm birth. bJog: an inter-national Journal of obstetrics & gynaecol-ogy 2002 Jun;109(6):718-720.
(49) neilson Jp. Ultrasound for fetal assessment in early pregnancy. cochrane Database of systematic reviews 2009;1.
(50) ananth cV, peltier mr, Kinzler wl, smulian Jc, Vintzileos am. chronic hypertension and risk of placental abruption: is the associa-tion modified by ischemic placental disease? american Journal of obstetrics & gynecol-ogy 2007 09;197(3):273-277.
(51) westfall rE, benoit c. the rhetoric of “natural” in natural childbirth: childbearing women’s perspectives on prolonged preg-nancy and induction of labour. soc.sci.med. 2004 oct;59(7):1397-1408.
(52) berghella V, rogers ra, lescale K. stripping of membranes as a safe method to reduce prolonged pregnancies. obstet.gynecol. 1996 Jun;87(6):927-931.
(53) magann EF, chauhan sp, nevils bg, mcnamara mF, Kinsella mJ, morrison Jc. management of pregnancies beyond forty-one weeks’ gestation with an unfavor-able cervix. am.J.obstet.gynecol. 1998 Jun;178(6):1279-1287.
(54) cammu h, haitsma V. sweeping of the mem-branes at 39 weeks in nulliparous women: a randomised controlled trial. br.J.obstet.gynaecol. 1998 Jan;105(1):41-44.
(55) de miranda E, van der bom Jg, bonsel gJ, bleker op, rosendaal Fr. membrane sweep-ing and prevention of post-term pregnancy in low-risk pregnancies: a randomised controlled trial. bJog : an international journal of obstetrics and gynaecology 2006
apr;113(4):402-408.
(56) gupta r, Vasishta K, sawhney h, ray p. safety and efficacy of stripping of mem-branes at term. int.J.gynaecol.obstet. 1998 Feb;60(2):115-121.
(57) Dare Fo, oboro Vo. the role of membrane stripping in prevention of post-term preg-nancy: a randomised clinical trial in ile-ife, nigeria. Journal of obstetrics & gynaecol-ogy 2002;22(3):283-286.
(58) boulvain m, stan cm, irion o. membrane sweeping for induction of labour. cochrane Database of systematic reviews 2009;1.
(59) Dove D, Johnson p. oral evening primrose oil: its effect on length of pregnancy and selected intrapartum outcomes in low-risk nulliparous women. J.nurse.midwifery 1999 may-Jun;44(3):320-324.
(60) wedig KE, whitsett Ja. Down the primrose path: petechiae in a neonate Exposed to herbal remedy for parturition. J.pediatr. 2008 January, 2008;152(1):140-e1.
(61) smith ca, crowther ca. acupuncture for induction of labour. cochrane Database of systematic reviews 2009;1.
(62) smith ca. homoeopathy for induction of labour. cochrane Database of systematic reviews 2009;1.
(63) woods nc. postdate pregnancy: part ii clini-cal implications. J nur midwif 1985;30(5).
(64) wennerholm Ub, hagberg h, brorsson b, bergh c. induction of labor versus expect-ant management for post-date pregnancy: is there sufficient evidence for a change in clinical practice? acta obstet.gynecol.scand. 2009;88(1):6-17.
(65) rayburn wF. minimizing the risks from elec-tive induction of labor. [review] [30 refs]. J.reprod.med. 2007 08;52(8):671-676.
(66) porreco rp. meeting the challenge of the rising cesarean birth rate. obstet.gynecol. 1990 Jan;75(1):133-136.
(67) rayburn wF, Zhang J. rising rates of labor in-duction: present concerns and future strate-gies. obstet.gynecol. 2002 Jul;100(1):164-167.
(68) perinatal partnership program of Eastern and southeastern ontario (ppEso). annual perinatal statistical report 2007-08. 2008.
(69) luckas m, bricker l. intravenous prosta-glandin for induction of labour. cochrane Database of systematic reviews 2009;1.
(70) gulmezoglu am, crowther ca, middleton p. induction of labour for improving birth outcomes for women at or beyond term. cochrane Database of systematic reviews 2009;1.
(71) pavicic h, hamelin K, menticoglou sm. Does routine induction of labour at 41 weeks really reduce the rate of caesarean section compared with Expectant managment? Jogc 2009;31(7):621.
(72) Duff c, sinclair m. Exploring the risks associ-ated with induction of labour: a retrospec-
tive study using the nimats database. northern ireland maternity system. J.adv.nurs. 2000 Feb;31(2):410-417.
(73) Demissie K, Joseph Ks, Dzakpasu s editors. perinatal health indicators for canada. ot-tawa: minister of public works and govern-ment services canada; 2000.
(74) parry E, parry D, pattison n. induction of labour for post term pregnancy: an obser-vational study. aust.n.Z.J.obstet.gynaecol. 1998 aug;38(3):275-280.
(75) alexander Jm, mcintire DD, leveno KJ. Forty weeks and beyond: pregnancy outcomes by week of gestation. obstet.gynecol. 2000 aug;96(2):291-294.
(76) menticoglou sm, hall pF. routine induction of labour at 41 weeks’ gestation: nonsen-sus consensus. bJog: an international Journal of obstetrics & gynaecology 2002 may;109(5):485-491.
(77) luckas m, buckett w, alfirevic Z. comparison of outcomes in uncomplicated term and post-term pregnancy following spontane-ous labor. J.perinat.med. 1998;26(6):475-479.
(78) ryo E, Kozuma s, sultana J, Kikuchi a, Fujii t, Unno n, et al. Fetal size as a determinant of obstetrical outcome of post-term pregnan-cy. gynecol.obstet.invest. 1999;47(3):172-176.
(79) li t, rhoads gg, Demissie K, smulian J. the efficacy of the non-stress test in preventing fetal death in post-term pregnancy. pae-diatr.perinat.Epidemiol. 2001 Jul;15(3):265-270.
(80) locatelli a, Zagarella a, toso l, assi F, ghidini a, biffi a. serial assessment of amniotic fluid index in uncomplicated term pregnancies:prognostic value of amniotic fluid reduction. Journal of maternal-Fetal & neonatal medicine 2004 04;15(4):233-236.
(81) morris Jm, thompson K, smithey J, gaffney g, cooke i, chamberlain p, et al. the useful-ness of ultrasound assessment of amni-otic fluid in predicting adverse outcome in prolonged pregnancy: a prospective blinded observational study. bJog : an international journal of obstetrics and gynaecology 2003 nov;110(11):989-994.
(82) Divon my, marks aD, henderson cE. longitu-dinal measurement of amniotic fluid index in postterm pregnancies and its association with fetal outcome. am.J.obstet.gynecol. 1995 Jan;172(1 pt 1):142-146.
(83) bresadola m, lo mastro F, arena V, bel-laveglia l, Di gennaro D. prognostic value of biophysical profile score in post-date pregnancy. clin.Exp.obstet.gynecol. 1995;22(4):330-338.
(84) heazell aE, Froen JF. methods of fetal movement counting and the detection of fetal compromise. J.obstet.gynaecol. 2008 Feb;28(2):147-154.
(85) Froen JF, heazell aE, tveit JV, saastad E, Fretts rc, Flenady V. Fetal movement
Clinical Practice Guideline: Pregnancy Beyond 41+0 Weeks 21
assessment. semin.perinatol. 2008 aug;32(4):243-246.
(86) grant a, Elbourne D, Valentin l, alexander s. routine formal fetal movement count-ing and risk of antepartum late death in normally formed singletons. lancet 1989 aug 12;2(8659):345-349.
(87) chanrachakul b, herabutya y. postterm with favorable cervix: is induction necessary?. Eur.J.obstet.gynecol.reprod.biol. 2003 Feb 10;106(2):154-157.
(88) roach VJ, rogers ms. pregnancy outcome beyond 41 weeks’ gestation. int.J.gynaecol.obstet. 1997 oct;59(1):19-24.
22 Association of Ontario Midwives